C nanocomposites with LaOCl nanoparticles embedded in carbon matrix

C nanocomposites with LaOCl nanoparticles embedded in carbon matrix

Journal of Alloys and Compounds 764 (2018) 701e708 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 764 (2018) 701e708

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Excellent microwave absorption of lamellar LaOCl/C nanocomposites with LaOCl nanoparticles embedded in carbon matrix Xiaolei Wang*, Xiukun Bao, Xinao Zhou, Guimei Shi Department of Chemistry and Environment, School of Science, Shenyang University of Technology, Shenyang, 110870, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 April 2018 Received in revised form 31 May 2018 Accepted 14 June 2018 Available online 18 June 2018

Lamellar LaOCl/C nanocomposites with LaOCl nanoparticles embedded in carbon matrix have been prepared by a facile and practical approach through the pyrolysis of LaCl3$7H2O with the assistance of NaCl for the formation of LaOCl nanoparticles and sequent in situ carbonization of glucose in colloidal mixture of LaOCl and glucose. Compared with dielectric loss factors of LaOCl nanoparticles, lamellar LaOCl/C nanocomposites present enhanced multiple-dielectric resonance behaviors, which are attributed to the increase of carbon content, and thus exhibit excellent microwave absorption properties in 1 e18 GHz range due to proper impedance match and improved microwave attenuation capacity. The optimal reflection loss (RL) is 32 dB at 18 GHz with a thickness of 1.5 mm, and effective bandwidth with RL below 20 dB is 14.8 GHz covering 3.2e18 GHz at the integrated thickness of 1.5e6 mm, demonstrating that lamellar LaOCl/C nanocomposites would be promising absorbents in microwave absorption field. © 2018 Elsevier B.V. All rights reserved.

Keywords: LaOCl/C Electromagnetic Dielectric Microwave absorption Interfacial polarization

1. Introduction Recently, considerable attention has been intensively devoted to design and fabrication of microwave absorption materials in GHz frequency range for solving electromagnetic interfere/compatibility problems due to rapid development of electromagnetic technology in civil and military field [1e5]. In the past decade, various kinds of microwave absorbents based on magnetic and/or dielectric loss mechanism have been successfully prepared and exhibit excellent microwave absorption properties, which are highly relative to the complex permittivity and permeability, component, impedance match, and microstructure of microwave absorbents [6e10]. The excellent microwave absorbents generally have the characteristic of strong absorption ability, broad absorption bandwidth, thin matching thickness, and light weight. Nanocomposites comprised of magnetic and dielectric components with unique microstructure have been accepted as high-efficient microwave absorbents owing to effective complementarity of dielectric and magnetic loss, such as yolk-shell structure of [email protected] [11] and [email protected] [12], core-shell structure of [email protected]@BaTiO3 [13], [email protected] [14], [email protected] [15], and [email protected] [16], one dimensional structure of [email protected]

* Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.jallcom.2018.06.150 0925-8388/© 2018 Elsevier B.V. All rights reserved.

nanochains [17], [email protected] nanotube [18], and [email protected] nanorods [19], and two dimensional structure of porous [email protected] sheet [20] and [email protected]@[email protected] hierarchical structures [21]. Due to introduction of magnetic component, magnetic-based absorbents have to suffer from relative high density, easy oxidation, and poor dispersibility for preparation of coating, which could hinder the practicable application including non-magnetic work conditions and light microelectronic devices. Thus, it is still an urgent challenge for fabrication of non-magnetic microwave absorbents. Carbon materials such as graphite, carbon nanotubes and graphene, have poor microwave absorption performance due to the high electronic conductivity and the complex permittivity. More recently, significant development have been progressed in improving the microwave absorption properties of carbon materials. For example, graphene-wrapped ZnO hollow spheres were prepared by Yin and coworkers and the experiment results showed that the complex permittivity was decreased with the increase of ZnO amount and the optimal microwave absorption reached a maximum absorption of 45.05 dB at 9.7 GHz with a matching thickness of 2.2 mm [22]. Zhong and coworkers prepared chain-like carbon nanospheres and the maximum absorption reached 20.4 dB at 13 GHz and wide bandwidth with reflection loss lower than 10 dB covered 2.9e18 GHz with a thickness of 1.5e5 mm [23]. Porous carbon fibers synthesized by Li and

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coworkers showed a maximum reflection loss of 31 dB at 9.7 GHz, which can be attributed to dielectric loss and interference of multiple-reflected microwave due to porous microstructures [24]. Yolk-shell [email protected] composites synthesized by Du and coworkers showed a maximum reflection loss of 34.8 at 15 GHz and bandwidth with reflection loss lower than 10 dB is 5.4 GHz [25]. Through the modification of dielectric component and/or microstructure design, carbon-based non-ferromagnetic microwave absorbents can achieve desirable dielectric loss with proper impedance match and excellent microwave absorption. P-type LaOCl semiconductor nanomaterials has intriguing physicochemical properties and potential applications in catalyst support, gas sensors and optoelectronic devices [26e28]. The crystalline structure of LaOCl is matlockite-type tetragonal structure with [LaO]þ cation and Cl anion layers aligning along the crystallographic c direction and La3þ ions are coordinated with four oxygen and five chloride ions. This structural characteristic endows LaOCl nanomaterials space charge polarization in the existence of electromagnetic field. Moreover, due to size effect, the surface of LaOCl nanomaterials has a certain content of defects, which would act as polarization center for electromagnetic response. Additionally, combination LaOCl nanomaterials with carbon materials, not simple physical mixture, can yield interfacial polarization, which is necessary for dielectric loss and microwave absorption. Furthermore, lamellar carbon matrix can provide numerous conductive networks for electron transport, which is prone to resistance loss. Thus, lamellar LaOCl/C nanocomposites with LaOCl nanoparticles embedded in carbon matrix should have excellent microwave absorption performance. In our previous work [29], LaOCl/C composites were synthesized by a sol-gel method and exhibited relatively poor microwave absorption properties, because it is difficult to control the content of carbon component in LaOCl/C composites due to the limitation of synthesis method. In the present work, a facile and practical strategy for synthesis of lamellar LaOCl/C nanocomposites with LaOCl nanoparticles embedded in carbon matrix has been proposed by two-step processes. LaOCl nanoparticles were firstly prepared from the pyrolysis of LaCl3$7H2O with the assistance of NaCl and lamellar LaOCl/C nanocomposites were further synthesized through in situ carbonization of glucose under nitrogen atmosphere. The electromagnetic and microwave absorption properties of LaOCl nanoparticles and lamellar LaOCl/C nanocomposites were detailedly investigated. Compared with LaOCl nanoparticles, the microwave absorption properties of lamellar LaOCl/C nanocomposites were remarkably enhanced due to enhanced dielectric loss and proper impedance match.

2.2. Preparation of LaOCl/C composites 0.1 g of as-prepared LaOCl nanoparticles was dissolved in 40 mL of glucose solution with appropriate amount under continuous magnetic stirring for 30 min at room temperature. The suspension was then heated at 70  C for volatilization of distilled water to gain the colloidal mixture. After that, the colloidal mixture was further treated at 700  C for 4 h in nitrogen atmosphere to construct lamellar LaOCl/C nanocomposites. The usage of glucose in the synthesis process is 0.1 g, 0.7 g, and 1.2 g, corresponding to the sample of LC1, LC2, and LC3, respectively. 2.3. Characterization The crystalline structure of the resultant samples were demonstrated by means of X-ray diffraction (XRD) on a D/Max 2200 diffractometer with Cu Ka radiation (l ¼ 0.15406 nm). Raman spectra were identified over a Confocal Raman Microscope using 633 nm laser. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was performed on a Mettler Toledo thermal analyzer in the temperature range of room temperature to 700  C at a heating rate of 10  C/min. The morphology and size distribution of the samples were shown by a JEOL-6490 scanning electron microscopy (SEM) with accelerating voltage of 20 kV and a JEOL-2100F transmission electron microscopy (TEM) operated at an accelerating voltage of 200 kV. For the preparation of microwave absorbents, the samples were uniform mixed with paraffin (3:2 in weight ratio) and the mixture were then pressed into cylinder with an inner diameter of 3.04 mm, an outer diameter of 7.00 mm, and a thickness of 2 mm. The electromagnetic parameters were measured in the frequency range of 1e18 GHz by a vector network analyzer (Agilent E5071C) and reflection loss (RL) was calculated according to the transmission line theory. 3. Results and discussion

2. Experimental section

The synthesis processes of LaOCl nanoparticles and lamellar LaOCl/C nanocomposites are schematically depicted in Fig. 1. Homogeneous mixture of LaCl3$7H2O and NaCl derived from evaporation of the solution were heat treated at 700  C in air for 4 h for the decomposition of LaCl3$7H2O into LaOCl nanoparticles (Step 1). NaCl in the present system can act as an inhibitor to prevent the coarsening and aggregation of LaOCl nanoparticles. LaOCl nanoparticles were further obtained by dissolution of NaCl (Step 2). After that, the colloidal mixture of LaOCl and glucose were prepared by evaporation of their solution (Step 3) and then heated at 700  C for 4 h in nitrogen atmosphere for the carbonization of glucose monomers to synthesize the lamellar LaOCl/C composites with LaOCl nanoparticles embedded in carbon matrix (Step 4). The chemical reaction mechanism for formation of lamellar LaOCl/C composites can be described as the following equations:

2.1. Preparation of LaOCl nanoparticles

LaCl3 þ 2H2 O/LaðOHÞ2 Cl þ 2HCl

All chemical reagents in analysis grade were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received without any purification. A typical process for preparation of LaOCl nanoparticles, 0.8 g of LaCl3$7H2O and 0.2 g of NaCl were homogeneously dissolved in 30 mL of distilled water with ultrasonication for 1 min at room temperature and the resultant transparent solution were then heated at 70  C for slow volatilization of solvent to obtain the colloidal mixture. The colloidal mixture was further heated at 700  C for 4 h in air. The obtained samples were dissolved in distilled water and washed by ethanol for several times and finally dried at 45  C overnight.

700 C

LaðOHÞ2 Cl ! LaOCl þ H2 O Air

700 C

6LaOCl þ C6 H12 O6 ! 6LaOCl=C þ 6H2 O Nitrogen

(1) (2)

(3)

Fig. 2 presents the typical XRD patterns of LaOCl nanoparticles and LaOCl/C nanocomposites. As shown in Fig. 2, all the diffraction peaks of the samples can be well assigned to a pure tetragonal structure of LaOCl (JCPDS Card No. 08-0477). The intensive diffraction peaks confirm the high crystallization of LaOCl phase.

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Fig. 1. A schematic illustration for the synthetic strategy of LaOCl nanoparticles and LaOCl/C nanocomposites.

Fig. 2. XRD patterns of (a) LaOCl nanoparticles and (b) LC2 nanocomposites. Fig. 3. Raman spectra of LaOCL/C nanocomposites: (a) LC1, (b) LC2, and (c) LC3.

The average crystallite size of LaOCl phase can be estimated from the Scherrer equation, d ¼ kl/bcosq, where d is the grain size, k is a particle shape factor and taken as 0.9 for spherical nanoparticles, l is the wavelength of Cu Ka radiation, q is the corresponding Bragg angle, and b is the angular half-width of the diffraction peak at 2q. The average crystallite size of LaOCl phase in LaOCl nanoparticles and LC2 composites are estimated to be 32 nm and 34 nm, respectively. The similar crystallite size of two samples indicates that the LaOCl nanoparticles were not coarsening and well dispersed in carbon matrix during carbonization process. Furthermore, no lattice planes of carbon component can be detected in the XRD pattern, indicating that carbon components in the LaOCl/C composites have a relatively low graphitization degree. In order to determine the graphitization degree of carbon component in the LaOCl/C nanocomposites, the Raman spectrum of LaOCl/C nanocomposites with different usage of glucose was measured, as shown in Fig. 3. All of the LaOCl/C composites exhibit two obvious peaks in the range of 1000e1800 cm1. Characteristic D band and G band peaks are located at about 1323 and 1590 cm1, respectively, confirming the existence of carbon component in LaOCl/C nanocomposites. Generally, the D band indicates the disordered/finite size crystals of graphite (nanographite crystals) and the G band is active in perfect graphite. Thus, the intensity ratio

of D band to G band (ID/IG) can be used to evaluate the graphitization degree of carbon in LaOCl/C composite. All of ID/IG values in our samples are almost identical, which indicates that carbon components have the similar graphitization degree. Ferrari and Robertson have detailed investigated the variation of ID/IG values originating from carbonization degree of amorphous carbon to graphite and Du and coworkers' results also confirm the ID/IG values are highly sensitive to the carbonization temperature [30,31]. Thus, it is desirable that the similar graphitization degree in LaOCl/C nanocomposites with different usage of glucose can be obtained because of the same carbonization temperature in the present work. Moreover, the calculated values of ID/IG are ca.1, which is superior to other carbon-based microwave absorbents with smaller ID/IG values [19,23,25]. The relatively low graphitization-degree of carbon component is beneficial for the electromagnetic impedance matching. The amount of carbon components in LaOCl/C nanocomposites were determined by TGA and DSC at ambient pressure from room temperature to 700  C, as shown in Fig. 4. All of the samples show a slight loss weight deriving from the evaporation of absorbed water and a sharp loss weight originating from the oxidation of carbon

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Fig. 4. TGA (red line) and DSC (blue line) curves of the LaOCl/C nanocomposites: (a) LC1, (b) LC2, and (c) LC3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

component. With the increase of glucose usage, the carbon content in LaOCl/C nanocomposites can be increased and calculated to be 20 wt%, 56 wt%, and 65 wt%, corresponding to LC1, LC2, and LC3, respectively. Furthermore, the termination temperature of carbon oxidation and prominent DSC peaks shift to high temperature and DSC peaks gradually broaden, which further demonstrate the increase of carbon content in LaOCl/C nanocomposites [32]. The detailed microstructures of LaOCl/C nanocomposites were identified by SEM and TEM, as shown in Fig. 5. It is obvious that LaOCl nanoparticles comprise of spherical and spheroidic morphology with the average diameter of ca. 115 nm (Fig. 5(a) and (d)). This result demonstrates that LaOCl nanoparticles can be obtained in the present synthesis strategy and NaCl can effectively act as analogous surfactant and effectively restrain the growth and coarsening of LaOCl nanoparticles under high-temperature treatment process. Fig. 5(b) and (c) images reveal LaOCl nanoparticles are embedded in carbon matrix and less LaOCl nanoparticles can be observed with increasing thickness of carbon matrix through the increase of glucose usage. Representative TEM image of LC2 samples is shown in Fig. 5(e). LaOCl nanoparticles are well-dispersed in carbon matrix and not growth up during the carbonization process, in agreement with XRD results. Besides, carbon-layer overlapping on edge of carbon matrix can be observed, which confirms the increase of thickness of carbon layer due to improved carbon content and in accordance with the observation of SEM images. HRTEM images of carbon matrix in Fig. 5(f) indicate that there exist amorphous and nanographite carbon in carbon matrix. The interlayer distance of lattice fringe is calculated to be 0.34 nm, which agrees well with the (002) crystal plane of hexagonal graphite (JCPDS card no 75-1621) [6,33]. In short, lamellar LaOCl/C nanocomposites with LaOCl nanoparticles well-dispersed in carbon matrix have been successfully fabricated in the present method and the carbon content can be effectively regulated. According to electromagnetic theory, microwave absorption properties of microwave absorbents are highly associated with the electromagnetic parameters, i.e., the relative complex permittivity (εr ¼ ε0-jε00 ) and the relative complex permeability (mr ¼ m0 -jm00 ). The

real part (ε0 and m0 ) and imaginary part (ε00 and m00 ) symbolize the storage and loss capabilities of electric and magnetic energy of electromagnetic wave, respectively. The m0 and m00 of the asprepared samples are around 1 and 0, respectively, not shown here. Fig. 6 shows the frequency dependence of the complex permittivity of the as-prepared samples (60 wt%)-paraffin composites in the frequency range of 1e18 GHz. For LaOCl nanoparticles, ε0 and ε00 gradually decrease with the frequency increase along with slight fluctuation. Besides, relative-weak multiple dielectric-resonance peaks of LaOCl samples can be observed in ε00 -f curve, indicating dielectric relaxation process. Compared with LaOCl nanoparticles, both ε0 and ε00 of LaOCl/C nanocomposites are intensively enhanced and obvious multiple dielectric-resonance peaks can be achieved, which can be attributed to the relatively high electrical conductivity of carbon matrix and the increase of carbon content according to the free electron theory and effectivemedia theory. The higher values of ε0 and ε00 demonstrate that LaOCl/C nanocomposites show higher efficiency in storing and dissipating electric energy. Furthermore, both ε0 and ε00 of LaOCl, LC1and LC2 samples display a frequency dispersion behavior in the measured frequency range and the commonly observed in carbon based core-shell nanocomposites such as [email protected], [email protected], [email protected], [email protected] [32e35]. With the further increase of carbon content, ε00 of LC3 samples maintain at around 8 with more-intensive resonance peaks. These results demonstrate that carbon modification in lamellar LaOCl/C nanocomposites can effectively regulate complex permittivity. In order to analyze the polarization process in our samples, Debye dipolar relaxation process is accepted to illustrate the dielectric loss mechanism, which can be expressed as the following Equation [8,16,18].

    εs þ ε∞ 2  00 2 εs  ε∞ 2 ε0  þ ε ¼ 2 2

(4)

where, εs and ε∞ symbolize the static permittivity and the permittivity at high frequency limit, respectively. On basis of the above equation, the plot of ε00 versus ε0 is generally denoted as a cole-cole semicircle, corresponding to a Debye relaxation process. As shown in Fig. 7, LaOCl, LC1 and LC2 samples have the similar changeable tendency and exhibit four distorted cole-cole semicircles, which can be attributed to the appropriate adding content of carbon and synergistic effect of dielectric phase of LaOCl. Whereas, the ε00 -ε0 curve of LC3 samples exhibit distinguish difference with an intensive semicircle (labeled as 3), further confirming strong dielectric-resonant behaviors due to the further increase of conductivity. It should be noticed that the cole-cole semicircles of all the samples are somewhat distorted, revealing other polarization mechanism should exist in our samples. In the microwave frequency range, polarization process should mainly originate from dipolar polarization and interfacial polarization. Dielectric loss mechanism for our samples maybe explained as following: First, [LaO]þ cation, [Cl]- anion and surface defects in LaOCl nanoparticles can induce space charge polarization and dipolar polarization. Second, nanographite and defects in amorphous carbon can act as polarization center under electromagnetic irradiation. Third, charge accumulation at the interfacial region of LaOCl-C and Cparaffin due to different electronegativity can induce the interfacial polarization (Maxwell-Wagner relaxation process). Finally, lamellar carbon matrix is apt to form conductive network, which is beneficial to electron transport and resistance loss. Thus, Debye relaxation together with dipolar polarization, space charge polarization, interfacial polarization and resistance loss contribute to the dielectric loss. Furthermore, with the increase of carbon content,

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Fig. 5. SEM images of (a) LaOCl, (b) LC1 and (c) LC2 samples. TEM images of (d) LaOCl and (e) LC2 samples. HRTEM images of (f) carbon matrix in LC2 samples.

the amount of nanographite and defects of amorphous carbon in lamellar carbon matrix is improved, which enhances the dipolar polarization and resistance loss. Therefore, the cooperative effects of multiple dielectric loss mechanisms with variation of carbon content generate different dielectric relaxation processes. The microwave absorption properties of the samples (60 wt %)-paraffin composites in the frequency range of 1e18 GHz are stimulated by means of the complex permittivity and complex permeability according to the transmission-line theory, which can be described as the following formula [36e38]:

RL ¼ 20 logjðZin  Z0 Þ=ðZin þ Z0 Þj

(5)

i h Zin ¼ Z0 ðmr =εr Þ1=2 tanh jð2pfd=cÞðmr εr Þ1=2

(6)

where, Z0 is the impendence of free space, Zin is the input impendence, f is the microwave frequency, d is the thickness of the composites, c is the velocity of light. As shown in Fig. 8(a), LaOCl nanoparticles shows the maximum reflection loss (RL) of 8 dB at

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Fig. 6. Frequency dependence of the complex permittivity of the as-prepared samples (60 wt%)-paraffin composites: (a) real part (ε0 ) and (b) imaginary part (ε00 ).

Fig. 8. Microwave absorption properties of the as-prepared samples (60 wt%)- paraffin composites: (a) LaOCl, (b) LC1, (c) LC2, and (d) LC3.

Fig. 7. Cole-cole plots of the as-prepared samples (60 wt%)-paraffin composites: (a) LaOCl, (b) LC1, (c) LC2, and (d) LC3.

14.5 GHz with a matching thickness of 3 mm and the effective bandwidth (RL < 5 dB, corresponding to microwave absorption of 68%) is up to 13.5 GHz in the frequency range of 4.5e18 GHz by regulating the thickness of 3e6 mm. Compared with microwave absorption properties of LaOCl nanoparticles, that of lamellar LaOCl/C nanocomposites with the increase of carbon content can be effectively heightened, as shown in Fig. 8(b)-(d). Among lamellar LaOCl/C nanocomposites, the LC2 samples exhibits the best microwave absorption performance. All values of the RL peaks of LC2 samples with different thickness are in the vicinity of 30 dB. The maximum RL value can achieve 32 dB at 18 GHz with a thickness of 1.5 mm and absorption bandwidth below 20 dB (microwave absorption of 99%) is 14.8 GHz covering frequency range of 3.2e18 GHz with a thickness of 1.5e6 mm. Compared with typical carbon-based microwave absorbents shown in Table 1, lamellar LaOCl/C nanocomposites have not only a strong microwave absorption property and a thin matching thickness, but also broad absorption bandwidth. Therefore, lamellar LaOCl/C nanocomposites can be considered as a candidate for microwave absorption application.

As we know, excellent microwave absorbents should simultaneously possess impedance match and microwave attenuation capacity, which can be effectively regulated by the complex permittivity and permeability associated with the design of components and microstructures. According to the reported literatures [39,40], impedance matching degree can be valuated in terms of the absolute of difference of dielectric loss tangent and magnetic loss tangent, which can be expressed as △tangent loss ¼ rtandεtandmr. Smaller the △ value is, higher impedance matching degree is, indicating more incident wave can not reflection form the surface and permeate into the absorbents. Attenuation capacity can describe the microwave absorption ability of absorbents and be written as according to transmission line theory as following [9,40e42]:

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 00 00 00 00 00 00  ðm ε  m0 ε0 Þ þ ðm ε  m0 ε0 Þ2 þ ðm0 ε þ m ε0 Þ2 a¼ c (7) where, f is the frequency of electromagnetic wave and c is the velocity of light. As shown in Fig. 9, although LaOCl nanoparticles has the smallest △ value and the best impedance match, its attenuation capacity is low due to the weak dielectric loss, resulting the relative weakest absorption ability compared with LaOCl/C composites. After introduction of carbon component, dielectric loss abilities of lamellar LaOCl/C composites are remarkably enhanced and contribute to proper impedance match and improved microwave attenuation capacity. Compare among the LaOCl/C composites, △ value of LC3 gradually raise with the increase of frequency and higher than that of LC1 and LC2 in the 4.5e18 GHz range, leading to impedance mismatch and more reflection of incident wave from the surface of absorbents although its attenuation capacity is higher than that of others. For LC1 and LC2 samples, the similar impedance match and the relatively higher attenuation capacity of LC2 samples due to multiple-dielectric relaxation processes contribute the excellent microwave absorption properties.

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Table 1 Microwave absorption performances of carbon-based composites in previous reports and this work. Absorbents

RL tip (dB) and frequency (GHz)

Optimal thickness (mm)

Bandwidth (GHz) and integrated thickness (mm)

Ref.

[email protected] nanosheet Porous [email protected] nanorods [email protected]@[email protected] Graphene-ZnO hollow spheres Porous carbon fibers Yolk-shell [email protected] Fe/C nanocubes Graphene/[email protected]/ZnO Lamellar LaOCl/C

48.2 (6.4) 27.9 (14.96) 51.5 (14.6) 45.0 (9.7) 31.0 (9.7) 39.4 (16.5) 22 (13.6) 32.5 (14.0) 32 (18)

2.1 2.0 1.8 2.2 2.3 1.85 2.0 2.5 1.5

9.5 and 1.5e4.0 (RL < 10 dB) 10.5 and 2.0e5.0 (RL < 18 dB) 5.1 and 1.8 (RL < 10 dB) 3.3 and 2.2 (RL < 10 dB) 4.2 and 2.3 (RL < 5 dB) 13.5 and 1.0e5.0 (RL < 20 dB) 14.6 and 2.0e5.0 (RL < 10 dB) 9.0 and 2.0e5.0 (RL < 20 dB) 14.8 and 1.5e6 (RL < 20 dB)

[6] [19] [21] [22] [24] [25] [33] [39] here

Fig. 9. (a) △tangent loss and (b) Attenuation capacity of the as-prepared samples (60 wt%)-paraffin composites.

4. Conclusions In conclusion, lamellar LaOCl/C nanocomposites with LaOCl nanoparticles embedded in carbon matrix have been fabricated by a simple and high-efficient method. Experimental results and theoretical analysis reveal that the improved microwave absorption can be attributed to the enhancement of dielectric loss factors regulated by the proper increase of carbon content, deriving from synergistic effect of dielectric properties of LaOCl nanoparticles and carbon matrix as well as their lamellar microstructures. Consequently, lamellar LaOCl/C nanocomposites with carbon content of 56 wt% exhibit proper impedance match and strong microwave absorption. This research would provide a significant insight in design and optimize carbon-based non-magnetic microwave absorbents. Acknowledgements This work has been supported by National Natural Science Foundation of China under Grant No. 51601120 and Natural Science Foundation of Liaoning Province (20170540679). References [1] Y. Zhang, Y. Huang, T. Zhang, H. Chang, P. Xiao, H. Chen, et al., Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (2015) 2049e2053. [2] B. Wen, M. Cao, M. Lu, W. Cao, H. Shi, J. Liu, et al., Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures, Adv. Mater. 26 (2014) 3484e3489. [3] Y. Qing, W. Zhou, F. Luo, D. Zhu, Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber, Carbon 48 (2010) 4074e4080. [4] K. Singh, A. Ohlan, V.H. Pham, R. Balasubramaniyan, S. Varshney, J. Jang, et al., Nanostructured graphene/Fe3O4 incorporated polyaniline as a high performance shield against electromagnetic pollution, Nanoscale 5 (2013) 2411e2420. [5] Y.F. Zhou, L. Zhang, T. Natsuki, Y.Q. Fu, Q.Q. Ni, Facile synthesis of BaTiO3 nanotubes and their microwave absorption properties, ACS Appl. Mater. Interfaces 4 (2012) 2101e2106.

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